Protection circuit apparatus

ABSTRACT

A protection circuit apparatus comprises a retriggerable fuse having an awakened state and a sleep state. The retriggerable fuse has an input and an output and is arranged, when in the awakened state, to selectively prevent a load current from flowing from the input to the output in response to the load current exceeding a first current threshold. An activation circuit is also provided and is arranged to sense the load current being drawn. The activation circuit causes the retriggerable fuse to enter the awakened state from the sleep state when the sensed load current exceeds a second current threshold.

FIELD OF THE INVENTION

This invention relates to a protection circuit apparatus of the typethat, for example, comprises a retriggerable fuse having an awakenedstate and a sleep state, the retriggerable fuse being capable, when inthe awakened state, of selectively permitting a load current to flowtherethrough.

BACKGROUND OF THE INVENTION

In the field of circuit protection, particularly in the automotiveindustry, it has traditionally been the case that an electronic circuitconstituting a load, for example an audio entertainment system, coupledto a power supply system of an automobile has been protected by metallicfuses employing so-called “fuse wire” or metal that bridges a pair ofcontacts and melts when a current flowing through the wire exceeds amaximum current rating associated with the metal. Such fuses areprovided in disposable packages that can be plugged and unplugged into afuse board within a vehicle, for example so-called “blade” fuses. Inthis respect, such disposable fuses, in addition to beingenvironmentally unfriendly, require the express provision of the fuseboard within the vehicle that has to be easily accessible in order toreplace one or more fuses; this has a limiting effect upon automotivedesign. Furthermore, the need to replace fuses is inconvenient and canresult in engineering time to replace the fuses, resulting in highermaintenance costs for the vehicle as well as an increased cost in termsof consumables to an owner of the vehicle.

A resettable or retriggerable “fuse” has therefore been proposed as analternative to traditional disposable metallic fuses. In essence, theretriggerable “fuse” is, in fact, a circuit providing current shut-offfunctionality in over-current situations. Such retriggerable “fuse”circuits, known as protected relay circuits, typically employ ann-channel FET as a way of protecting a load against short-circuits.However, mechanical implementations that use mechanical alternatives tothe n-channel FET exist as well.

One particularly important requirement of protected relay circuits isthat they must be able to provide protection to a load in a number ofscenarios. For example, in addition to when the engine of the vehicle isrunning, the protected relay circuit needs to be in an active state andable to provide protection when the engine of the vehicle is notrunning, i.e. when the battery is not being recharged and power is notbeing provided by an alternator of the vehicle. Of course, vehiclemanufacturers naturally place constraints upon power consumption byelectronic devices in the vehicle when the engine is not running as thecharge of the battery must not be unnecessarily drained. A number ofloads in the vehicle need to draw small amounts of current, in the orderof a few tens of micro-Amps, whilst the engine is not running, forexample: an electronic clock and a central locking, alarm and engineimmobiliser system, to name but a few.

In relation to certain electronic equipment in the vehicle, theoperative state of the equipment can change, for example through userinteraction, resulting in the circuits of the equipment drawing greateramounts of current (for example, up to a few Amps) without warning. Thecentral locking, alarm and engine immobiliser system and audio (andpossibly audiovisual) entertainment equipment in the vehicle areexamples of such equipment.

However, since flow of a load current drawn by the equipment to beprotected is relatively high, the protected relay circuit used has tohave a relatively low “on” resistance, for example a low Drain-Source“on” resistance (RDSon) for FET implementations, in order to reducepower dissipating by the protected relay circuits. Additionally, suchprotected relay circuits require a constant bias current, which isunacceptably high, in order to switch the protected relay circuit to an“on” state and provide the necessary protection against over-currentevents.

Further, in order to achieve the low RDSon mentioned above, n-channelFET-based protected relay circuits require a charge pump, or periodicgate refresh circuitry. However, the provision of the charge pump orperiodic gate refresh circuitry requires the protected relay circuit todraw an unacceptably high level of current in the order of at least 100μA. As will be appreciated, repeated instances of the n-channel FETprotected relay circuit to replace all or almost all fuses in thevehicle that protect the various loads contained in the vehicle withknown high-side FET protected relay circuits will result in anunacceptably high current drain on the battery of the vehicle.

In contrast, p-channel FET implementations of the protected relaycircuit can have reduced current requirements to drive a gate of theFET, but have higher financial costs associated with their use (due toincreased die area requirements over n-channel implementations) and alsorequire an unacceptably high bias current to provide the necessaryprotection required. Consequently, p-channel FET implementations are notwidely employed. Also, most integrated circuit technologies andtechniques are optimised for circuits employing power n-channel FETs.Additionally, p-channel FET implementations are difficult to design toachieve levels of protection comparable to those of n-channel FETs interms of accuracy of threshold implementation for short-circuitprotection.

As can be seen from the above-described known technologies, in somecircumstances protected relay circuits consume unacceptably largeamounts of current to provide accurate over-current protection and soare sub-optimal implementations from the point of view of vehiclemanufacturers.

STATEMENT OF INVENTION

According to the present invention, there is provided a protectioncircuit apparatus as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

At least one embodiment of the invention will now be described, by wayof example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram of a protection circuit apparatusconstituting an embodiment of the invention; and

FIG. 2 is a flow diagram of operation of the circuit of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout the following description identical reference numerals willbe used to identify like parts.

Referring to FIG. 1, a protection circuit apparatus 100 comprises aretriggerable fuse 102 and an activation circuit 104. In this example,the retriggerable fuse 102 is a solid state relay, but can be anysuitable protected relay circuit or the like implemented electronically,mechanically or as a combination of the two. For example, theretriggerable fuse 102 can be any suitable electronic switching deviceor circuit capable of providing protection from over-current events. Theretriggerable fuse 102 comprises an n-channel high-side FET switch 106,the FET 106 having a gate terminal and a drain terminal coupled to agate driver 108 comprising a charge pump. The gate driver 108 is anysuitable known circuit for driving the FET 106 or other device if usedin place of the FET 106. The drain terminal of the FET 106 is coupled toa supply rail 110 and constitutes a current input 112.

The gate driver 108 is coupled to a control circuit 114 for biasing,controlling and protecting the FET 106, the control circuit 114 alsobeing coupled to over-current circuitry 115 for detecting anover-current event. The control circuit 114 has a control input 116 anda wake-up, or awaken, input 118. Of course, in another embodiment, thecontrol input 116 and the awaken input 118 can be provided as a singleinput.

A source terminal of the FET 106 constitutes a current output 120, aconnection being provided to a current sense circuit (not shown), forexample a supplementary sense FET operably coupled to the FET 106 aspart of a current mirror circuit, in order to provide a current-senseoutput 122 of the retriggerable fuse 102. The current-sense output 122is also coupled to the over-current circuitry 115. A housing (not shown)of the retriggerable fuse 102 is coupled to ground potential 124.Additionally, the current-sense output 122 is coupled to the groundpotential 124 via a zener diode 125. The supply rail 110 is also coupledto the ground potential 124 via, in this example, an automotive battery126.

The activation circuit 104 comprises a first capacitor 128 coupledbetween the supply rail 110 and the current output 120. Similarly, afirst resistor 130 is coupled in parallel with the first capacitor 128between the supply rail 110 and the current output 120. A secondresistor 132 has a first terminal coupled to the current output 120 anda second terminal coupled to a base terminal of a PNP bipolar transistor134; the current output 120 is also coupled to a load 136, the load 136also being coupled to the ground potential 124. Of course, the skilledperson will appreciate that any threshold-dependent conduction devicecan be employed and that the PNP bipolar transistor 134 is just oneexample of such a device. In this respect, the first resistor 130, thesecond resistor 132 and the threshold-dependent conduction device can bereplaced by a comparator-based circuit. The first resistor 130 can bereplaced by any other suitable known device or devices capable ofconducting small amounts of current, for example a transistor, such asan NPN, PNP, NMOS or PMOS transistor, or an integrated circuit. In onealternative embodiment, a self-biased PNP transistor, i.e. configured asa diode, can replace the first and second resistors 130, 132 and becoupled to form a current mirror arrangement in order to monitor thecurrent drawn by the load 136. In such an embodiment, another sense FETcan be coupled to the FET 106 and the self-biased PNP transistormentioned above.

In this example, the protection circuit apparatus 100 is being used forautomotive applications and so the load 136, in this example, can be anysuitable electronic equipment, for example an audio entertainmentsystem, a digital clock, or an engine immobiliser, central locking andalarm system.

The bipolar transistor 134 has an emitter terminal coupled to the supplyrail 110 and a collector terminal coupled to a first terminal of a thirdresistor 137. A second terminal of the third resistor 137 is coupled tofirst terminals of a fourth resistor 138 and a fifth resistor 140 aswell as a first terminal of a sixth resistor 142.

A second terminal of the fourth resistor 138 is coupled to the awakeninput 118 of the retriggerable fuse 102, a second capacitor 144 beingcoupled between the second terminal of the fourth resistor 138 and asecond terminal of the sixth resistor 142. The second terminal of thesixth resistor 142 is also coupled to the ground potential 124. A secondterminal of the fifth resistor 140 is coupled to the control input 116of the retriggerable fuse 102, a third capacitor 146 being coupledbetween the second terminal of the fifth resistor 140 and the groundpotential 124.

In operation (FIG. 2), the apparatus 100 is powered up. In order toexplain operation of the apparatus 100 in greater detail, the apparatus100 will now be described in the context of the load 136 being anin-vehicle audio entertainment system (not shown). However, the skilledperson will appreciate that other loads can be employed in place of thein-vehicle audio entertainment system.

Initially, in this example, the load 136 is in a “standby” state anddrawing a small load current, I_(load), to power a clock (also notshown) provided with the in-vehicle audio entertainment system.Additionally, the retriggerable fuse 102 is in a sleep state havinglogic 0 signals at the control input 116 and the awaken input 118. Onlya very small leakage current smaller than 1 μA, is drawn by theapparatus 100. However, it can be seen that although the retriggerablefuse 102 is in the sleep state, an alternative current path to the load136 runs via the first resistor 130, because no current is permitted, inthis example, to flow through the FET 106 when the retriggerable fuse102 is in the sleep state. Additionally, the path to the load 136provided via the first resistor 130 serves to allow the current drawn bythe load 136 to be monitored. In this respect, as the load current,I_(load), drawn by the load 136 is small, the first resistor 130 doesnot develop a sufficiently large detectable voltage, V_(R1), acrossfirst and second terminals thereof and so the detectable voltage,V_(R1), does not exceed a base-emitter voltage, V_(be), of the bipolartransistor 134 (Step 200). Consequently, the bipolar transistor 134 isin an “off” state, i.e. not conducting current. The base-emittervoltage, V_(be), constitutes a threshold voltage, above which theretriggerable fuse 102 has to be placed in an awakened state. Thisthreshold voltage is associated with a threshold wake-up current,I_(wake), above which the bipolar transistor 134 has to transition to an“on” state in order to change the operative state of the retriggerablefuse 102.

In the event that the in-vehicle audio entertainment system, i.e. theload 136 in this example, is switched into an “on” state from thestandby state, the load current, I_(load), drawn by the load 136increases to, for example, 5A and exceeds the wake-up current, I_(wake),threshold. Consequently, the detectable voltage, V_(R1), developedacross the first resistor 130 exceeds the base-emitter voltage, V_(be),of the bipolar transistor 134. Hence, the load current, I_(load), beinggreater than the wake-up current, I_(wake), associated with the standbystate, has been detected. As the detectable voltage, V_(R1), is greaterthan the base-emitter voltage, V_(be), of the bipolar transistor, thebipolar transistor 134 transitions into an “on” state and conductscurrent therethrough. Generation of an enable signal (Step 202) by thebipolar transistor 134 therefore results and current flows through thesixth resistor 142, resulting in the generation of a logic 1 “enable”voltage signal applied to the control input 116 and the awaken input118. The second and third resistors 132, 137 serve to protect thebipolar transistor 134 and the control and awaken inputs 116, 118,respectively.

In response to the enable signal being applied at the control input 116and the awaken input 118, the retriggerable fuse 102 under the controlof the control circuit 114 detects (Step 204) the presence of the enablesignal and enters the awakened state and switches the FET 106 into an“on” state, the load current, I_(load), then beginning to flow throughthe FET 106. Consequently, current flowing through the first resistor130 diminishes and so the detect voltage, V_(R1), no longer exceeds thebase-emitter voltage, V_(be), of the bipolar transistor 134. The enablesignal, generated by the bipolar transistor 134, the third resistor 137and the sixth resister 142 in combination, therefore stops beinggenerated and hence applied to the control input 116 and the awakeninput 118. However, since the retriggerable fuse 102 is in the awakenedstate and the load current, I_(load), is flowing through the FET 106,the retriggerable fuse 102 is detecting the load current, I_(load), andgenerating a current-sense signal, I_(sense), at the current-senseoutput 122 (Step 206). The current-sense signal, I_(sense), is, in thisexample, proportional to (but considerably smaller than) the loadcurrent, I_(load), but in any event indicative that the load current,I_(load), has been sensed. In this respect, the current-sense signal,I_(sense), is an attenuated version of the load current, I_(load):I_(load)/β, where β is a recopy ratio of the sense FET relative to theFET 106.

When an awaken trigger voltage, V_(t), is applied at the control input116 and the awaken input 118, the retriggerable fuse 102 can remain inthe awakened state. Hence, when the current-sense signal, I_(sense),exceeds a threshold current level known as a “sustain” current,I_(sustain), level corresponding to:

$\frac{V_{t}\beta}{R_{6}},$where R₆ is the resistance of the sixth resistor 142, the retriggerablefuse 102 remains in the awakened state. In this example, thecurrent-sense signal, I_(sense), flows through the sixth resistor 142 ata level exceeding the sustain current, I_(sustain), level and somaintains the logic 1 voltage signal at the control input 116 and theawaken input 118 of the retriggerable fuse 102, thereby maintaining theretriggerable fuse 102 in the awakened state. Hence, it can be seen thatthe retriggerable fuse 102 can be latched in the awakened state throughuse of the current-sense signal, I_(sense), to generate the enablesignal.

Of course, the skilled person will appreciate that the retriggerablefuse 102 may already be in the awakened state, for example, the awakenedstate may automatically be entered when an engine of a vehicle isstarted.

Once the load 136 returns to the standby state, the load current,I_(load), flowing through the FET 106 diminishes and so thecurrent-sense signal, I_(sense), also diminishes below the sustaincurrent, I_(sustain), level necessary to maintain the enable voltagesignal at the control and awaken inputs 116, 118. Additionally, thecurrent flowing through the first resistor 130 is below the wake-upcurrent, I_(wake), threshold and so is insufficient to result in thedetectable voltage, V_(R1), exceeding the base-emitter voltage, V_(be),of the bipolar transistor 134.

Consequently, the enable signal cannot be generated and hence be presentat the control input 116 and the awaken input 118, resulting in theretriggerable fuse 102 reverting to the sleep state (not shown in FIG.2).

In contrast, in the event that the load current, I_(load), increases toor exceeds a level where an over-current event is considered to havetaken place, for example, as a result of a short-circuit (Step 208), theover-current circuitry 115 detects the over-current event, and using thecontrol circuit 114, instructs the gate driver 108 to place the FET 106in an “off” state (Step 210). The instruction to turn the FET 106 off isin response to the load current, I_(load), exceeding an overcurrentthreshold, I_(overcurrent), and is a latched signal that is not releaseduntil the retriggerable fuse 102 reverts to the sleep state. Theovercurrent threshold, I_(overcurrent), corresponds to a determinationthat the over-current event is taking place, i.e. the load current hasreached or exceeded the level mentioned above in relation to theovercurrent event.

Consequently, the, now excessive, load current, I_(load), ceases to flowthrough the FET 106 and so protection is provided to the load 136. Thisis part of the normal known operating procedure of the retriggerablefuse 102. Of course, once the load current, I_(load), no longer flowsthrough the retriggerable fuse 102, the current-sense signal isextinguished (Step 212). However, the load current, I_(load), then flowsthrough the first resistor 130, the value of the first resistor 130being sufficiently high to sustain high voltages thereacross, andresults in the detect voltage, V_(R1), exceeding the base-emittervoltage, V_(be), and so the bipolar transistor 134, the third resistor136 and the sixth resistor 142 operating in combination, resumegenerating the enable signal, thereby retaining the retriggerable fuse102 in the awakened state with the FET 106 in the “off” state.

Once the short-circuit has been removed and the over-current event hasfinished, the load current, I_(load), returns to the level associatedwith the load 136 being in the “on” state. The retriggerable fuse 102therefore remains in the awakened state as the detect voltage, V_(R1),is still greater than the base-emitter voltage, V_(be), of the bipolartransistor 134. The FET 106 thus remains latched in the “off” state dueto the previous occurrence of the over-current event. However, if theload 136 is returned to the standby state, for example by a user, thedetect voltage, V_(R1), falls below the base-emitter voltage, V_(be), ofthe bipolar transistor 134 and so is insufficient to cause the enablesignal to be provided at the control and awaken inputs 116, 118 by theactivation circuit 104. Consequently, the retriggerable fuse 102 revertsto the sleep state, resulting in the latch signal no longer beingapplied by the over-current circuitry 115 to the gate driver 108 via thecontrol circuit 114.

A processing resource, for example a microcontroller, can be coupled tothe retriggerable fuse 102 in order to override the actions of theactivation circuit 104 in response to an external stimulus, for examplean override signal to instruct removal of the latch signal withoutneeding the load 136 to enter the standby state. In this respect, thefourth resistor 138 and the second capacitor 144, and the fifth resistor140 and the third capacitor 146 serve to “condition”, by limiting andfiltering, the enable signal that can be generated by the bipolartransistor 134 prior to use by the microcontroller.

Of course, the skilled person will appreciate that even if theretriggerable fuse 102 is in the sleep state when the over-current eventoccurs, the sudden presence of the excessive load current, I_(load), issufficient to cause the bipolar transistor 134 and the third resistor137 to generate the enable signal causing the retriggerable fuse 102 toenter the awakened state, detect that the overcurrent threshold,I_(overcurrent), has been exceeded and cause the FET 106 to stopconducting current, thereby protecting the load 136 from theover-current event.

In another embodiment, the first resistor 130 can be replaced bymodifying the configuration of the retriggerable fuse 102. Instead ofproviding the first resistor 130, the gate driver 108 can be modified sothat the FET 106 operates as a transdiode when the retriggerable fuse102 is in the sleep state and the potential of the battery 126 or aslightly higher potential can be applied to the gate of the FET 106. Inthis configuration, the small load current required to power, forexample, the clock of the in-vehicle audio entertainment system isprovided via the FET 106 instead of the first resistor 130 when theretriggerable fuse 102 is in the sleep state.

In this and other embodiments, the second resistor 132 is also notemployed and the current drawn by the load 136 can be measured bycoupling another PNP transistor to the bipolar transistor 134, asalready described above, to form a current mirror arrangement, thecurrent mirror being coupled to another sense FET (not shown) that is,in turn, coupled to the FET 106 in order to obtain a copy of currentflowing through the FET 106 when the retriggerable fuse 102 is in thesleep state.

Additionally or alternatively, the use of the sixth resistor 142 can beobviated by providing an open node signal generator. The open nodesignal generator can be provided by coupling a buffer between the outputof the current sense circuit of the retriggerable fuse 102 describedabove and the first terminals of the fourth and fifth resistors 138, 140so that the fourth and fifth resistors 138, 140 are disposed between anoutput of the buffer and the second and third capacitors 144, 146. Apull-down current source is also coupled to the input of the buffer inaddition to the output of the current-sense circuit and the zener diode125 can serve as a voltage clamp to ensure logic levels of, for example0V and 5V. Consequently, the sustain current, I_(sustain), level can beprovided as a digital output signal.

It is thus possible to provide a protection circuit apparatus capable ofreplacing use of a metallic fuse that uses fusible metal, whilst notdrawing undesirably high currents beyond those consumed by a load whenthe apparatus is not preventing over-currents from flowing. Theapparatus is also re-settable. Hence, load protection can be provided atall times. Additionally, by not re-setting the retriggerable fuse whentransitioning from the awakened state to the sleep state, knowledge ofan over-current event is retained, thereby avoiding damage being causedto the load when the over-current event is still in progress. Of course,the above advantages are exemplary, and these or other advantages may beachieved by embodiments of the invention. Further, the skilled personwill appreciate that not all advantages stated above are necessarilyachieved by embodiments described herein.

1. A protection circuit apparatus for protecting a load from anover-current, the apparatus comprising: a retriggerable fuse comprisinga control circuit, the retriggerable fuse having an awakened state and asleep state, the retriggerable fuse being capable, when in the awakenedstate, of selectively preventing a load current from flowing from aninput to an output thereof in response to the load current exceeding afirst current threshold; and an activation circuit arranged to sense theload current being drawn and cause the retriggerable fuse to enter theawakened state from the sleep state in response to the sensed loadcurrent exceeding a second current threshold; wherein the retriggerablefuse has a current-sense output to provide a current-sense output signalindicative that the load current has been sensed, the activation circuitbeing arranged with the control circuit to use the current-sense outputsignal to latch the retriggerable fuse in the awakened state, whereinthe current-sense output signal is proportional to the load current;wherein the retriggerable fuse reverting to the sleep state once thecurrent-sense output signal diminishes below a sustain current level soas to release the latch necessary to maintain the retriggerable fuse inthe awakened state.
 2. An apparatus as claimed in claim 1, wherein theload current is drawn when the retriggerable fuse is in the sleep stateand the load current is less than the second current threshold.
 3. Anapparatus as claimed in claim 2, wherein the load current is drawnthrough the retriggerable fuse.
 4. An apparatus as claimed in claim 3,wherein the retriggerable fuse comprises another threshold-dependentconduction device, the load current being drawn through the anotherthreshold-dependent conduction device.
 5. An apparatus as claimed inclaim 1, wherein the retriggerable fuse is arranged to attempt torelease the retriggerable fuse from being latched in the awakened statein response to the retriggerable fuse preventing, when in the awakenedstate, the load current from flowing from the input to the output of theretriggerable fuse.
 6. An apparatus as claimed in claim 5, wherein theactivation circuit is arranged to maintain the retriggerable fuse in theawakened state in response to the sensed load current continuing toexceed the second current threshold.
 7. An apparatus as claimed in claim6, wherein the retriggerable fuse is a solid-state relay.
 8. Anapparatus as claimed in claim 1, wherein the activation circuitcomprises a threshold-dependent conduction device for transitioning ormaintaining the retriggerable fuse in the awakened state, thethreshold-dependent conduction device being responsive to the loadcurrent drawn exceeding the second current threshold.
 9. An apparatus asclaimed in claim 8, wherein the threshold-dependent conduction device isa transistor.
 10. An apparatus as claimed in claim 9, wherein thetransistor is a bipolar transistor.
 11. An apparatus as claimed in claim8, wherein the threshold-dependent conduction device is a voltagecomparator.
 12. An apparatus as claimed in claim 1, further comprising:a processing resource coupled to the retriggerable fuse and arranged tooverride prevention of flow of the load current from the input to theoutput of the retriggerable fuse in response to a stimulus external tothe processing resource.
 13. A method for protecting a load from anover-current, the method comprising: detecting that a load current isabove a first threshold current; transitioning a first transistor to anon state when the load current is above the first threshold current;generating an enable signal via the first transistor when the firsttransistor is in the on state; transitioning a retriggerable fuse to anawaken state and a second transistor into an on state in response to theenable signal; providing the load current to the load through the secondtransistor when the second transistor is in the on state; generating acurrent-sense signal that is proportional to the load current via thesecond transistor; detecting that the load current has exceeded anovercurrent threshold based on the current-sense signal; placing thesecond transistor in an off state when the load current has exceeded anovercurrent threshold; ceasing a flow of the load current to the loadwhen the second transistor is in the off state; transitioning theretriggerable fuse in a sleep state when the load current is below thefirst current threshold; and enabling the flow of the load current tothe load when the load current is below a sustain current level.
 14. Themethod of claim 13, further comprising: placing the first transistor inan off state when the second transistor is in the on state; andtransitioning the first transistor to the on state when the load currentis above the overcurrent threshold.
 15. The method of claim 13, furthercomprising: maintaining the second transistor in the on state while thecurrent-sense signal is above the sustain current level.
 16. The methodof claim 13, further comprising: maintaining the retriggerable fuse inthe awaken state and the second transistor in the off state continue tocease the flow of the load current to the load while the load current isabove the overcurrent threshold.
 17. A protection circuit apparatus forprotecting a load from an over-current, the apparatus comprising: anactivation circuit including a first transistor transitions to an onstate and generates an enable signal when a load current is above afirst threshold current; and a retriggerable fuse including: a secondtransistor to provide the load current to the load when the secondtransistor is in an on state, to generate a current-sense signal that isproportional to the load current, to cease providing the load current tothe load when the second transistor is in an off state; and a controlcircuit in communication with the activation circuit and the secondtransistor, the control circuit to transition the retriggerable fuse toan awaken state and to transition the second transistor to the on statein response to receiving the enable signal, to maintain the secondtransistor in the on state while the current-sense signal is above asustain current level, to detect that the load current has exceeded anovercurrent threshold based on the current-sense signal, to place thesecond transistor in the off state when the load current has exceededthe overcurrent threshold, to maintain the retriggerable fuse in theawaken state and the second transistor in the off state while the loadcurrent has exceeded the overcurrent threshold, and to transition theretriggerable fuse to a sleep state when the load current is below thesustain current level, wherein the first transistor transitions to anoff state in response to the second transistor transitioning to the onstate.
 18. The protection circuit apparatus of claim 17, wherein theactivation circuit maintains the retriggerable fuse in the awakenedstate in response to the current-sense signal continuing to exceed thesustain current level.
 19. The protection circuit apparatus of claim 17,further comprising: a processing resource in communication with theretriggerable fuse, the processing resource overrides ceasing of theflow of the load current to the load through the second transistor inresponse to an override signal.
 20. The protection circuit apparatus ofclaim 19, wherein the override signal is external to the processingresource.